The invention comprises certain novel metal oxide materials which exhibit superconductivity at elevated temperatures and/or which are useful as electrodes or electrolytes in electrochemical cells, sensors, and as catalysts.
It is known that certain classes of metal oxide will exhibit the phenomenon of superconductivity below a particular critical temperature referred to as Tc. These include as prototypes BaPb2xe2x88x92xBixO3xe2x88x92d, Ba2xe2x88x92xSrxCuO4xe2x88x92d, YBa2Cu3O7xe2x88x92d as described in The Chemistry of High Temperature Superconductors, ed. by Nelson et al, American Chem. Soc. 1987, and Bi2Sr2CaCu3O5xe2x88x92d as described by Subramanian et al, Science 239, 1015 (1988). We have identified this last material as the n=2 member in a homologous series of approximate formula Bi2(Sr,Ca)n+1CunO2n+4+d, n=0, 1, 2, 3, . . . , obtained by inserting an additional layer of Ca and an additional square planar layer of CuO2 in order to obtain each higher member. These materials often exhibit intergrowth structures deriving from a number of these homologues as well as Bi substitution on the Sr and Ca sites. Tc is observed to rise as n increases from 1 to 2 to 3. The material YBa2Cu4O8+d has a layered structure similar to the n=2 member of this series Bi2Sr2CaCu2O8+d and we expect therefore that YBa2Cu4O8xe2x88x92d belongs to similar series. One such serial could be obtained by insertion of extra Yxe2x80x94CuO2 layers resulting in the series of materials RnBa2Cun+3O3.5+2.5nxe2x88x92d, n=1, 2, 3, . . . and another by insertion of extra Caxe2x80x94CuO2 layers resulting in the series RBa2CanCun+4O8+2nxe2x88x92d, n=1, 2, . . . By analogy it may be expected that Tc in these two series should rise with the value of n.
Binary, ternary or higher metal oxide materials containing as cations one or more alkali earth elements, such as these materials and having high oxygen-ion mobility may also be used as electrodes, electrolytes and sensors for electrochemical applications. The oxygen-ions will move through such an electrolyte material under an applied electrical field allowing the construction of oxygen pumps for catalysis and other oxidizing or reduction processes involving the supply or extraction of atomic oxygen. The oxygen-ions will also move through such an electrolyte material under a concentration gradient allowing the construction of batteries, fuel cells and oxygen monitors. For these materials to act effectively as electrolytes in such applications it is necessary that they have high oxygen-ion mobility through the atomic structure of the materials and at the same time have a low electronic conductivity so that the dominant current flow is by oxygen-ions and not electrons. For these materials to act effectively as electrodes in such applications it is necessary that they have a high electronic conductivity as well as a high oxygen-ion mobility so that electrons which are the current carried in the external circuit may couple to oxygen-ions which are the current carrier in the internal circuit. Electrochemical cells including fuel cells, batteries, electrolysis cells, oxygen pumps, oxidation catalysts and sensors are described in xe2x80x9cSuperionic Solidsxe2x80x9d by S Chandra (North Holland, Amsterdam 1981).
Solid electrolytes, otherwise known as fast-ion conductors or superionic conductors have self diffusion coefficients for one species of ion contained within their crystalline structure ranging from 10xe2x88x927 to 10xe2x88x925 cm2/sec. A diffusion coefficient of about 10xe2x88x925 m2/sec is comparable to that of the ions in a molten salt and thus represents the upper limit for ion mobility in a solid and is tantamount to the sublattice of that particular ion being molten within the rigid sublattice of the other ions present. Such high diffusion mobilities translate to electrical conductivities ranging from 10xe2x88x922 to 1 S/cm, the latter limit corresponding to that commonly found in molten salts. The n=0 member of the series Bi2+exe2x88x92xPbx(Sr,Ca)n+1xe2x88x92sCunO2n+4+d and various substituted derivatives are identified as solid electrolytes with high oxygen-ion mobility. The n=1, 2 and 3 members of the series may have high oxygen-ion mobility as well as high electron conductivity and thus are potentially applicable as electrode materials.
The invention provides certain novel metal oxide materials which exhibit superconductivity at low temperatures and/or which are useful in such electrode, electrolyte, cell and sensor applications, or as electrochemical catalysts.
In broad terms the invention comprises metal oxide materials within the formula
Rn+1xe2x88x92uxe2x88x92sAuMm+eCunOwxe2x80x83xe2x80x83(1)
where nxe2x89xa70 and n is an integer or a non-integer, 1xe2x89xa6mxe2x89xa62, 0xe2x89xa6sxe2x89xa60.4, 0xe2x89xa6exe2x89xa64, and 2n+(1/2)xe2x89xa6wxe2x89xa6(5/2)n+4, with the provisos that u is 2 for nxe2x89xa71, u is n+1 for 0xe2x89xa6n less than 1
and where
R and A are each any of or any combination of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sr, Ba, Li, Na, K, Rb or Cs,
M is any of or any combination of Cu, Bi, Sb, Pb, Tl or any other transition metal,
Cu is Cu or Cu partially substituted by any of or any combination of Bi, Sb, Pb, Ti or any other transition metal,
O is O or O partially substituted by any of N, P, S, Se, or F.
and wherein the structure of the materials is characterised by distorted or undistorted substantially square planar sheets of CuO2 when n greater than 0 and distorted or undistorted substantially square sheets of R for n greater than 1.
excluding where M is Bi, R is Ca and Sr, A is Sr and Ca, and s and e are 0 and where n=1, the material Bi2(Sr1xe2x88x92xCax)2CuO8xe2x88x92d with 0xe2x89xa6xxe2x89xa60.3, and where n=2 the material Bi2(Sr1xe2x88x92xCax)3Cu2O10xe2x88x92d with 0.6xe2x89xa6xxe2x89xa60.33
and excluding RBa2Cu3O7xe2x88x92d 
and excluding, where R is as above excluding Ca, Sr, Ba, Li, Na, K, Rb or Cs, the material having formula
RBa2Cu4O8xe2x88x92d.
The n=1, 2, 3, 4, 5, . . . materials have pseudo-tetragonal structures with lattice parameters a, b and c given by 5.3 xc3x85xe2x89xa6a,bxe2x89xa65.5 xc3x85 and c=18.3xc2x1v+(6.3xc2x1vxe2x80x2)n xc3x85 where 0xe2x89xa6v, vxe2x80x2xe2x89xa60.3. The n=0 material extends over the solubility range 0xe2x89xa6exe2x89xa64 and has orthorhombic or rhombohedral symmetry with lattice parameter c=19.1xc2x1v xc3x85.
Preferred materials of the invention are those of formula (1) wherein m is 2 and R is Ca and R is predominantly Bi and having the formula
xe2x80x83Bi2+exe2x88x92xLxCan+1xe2x88x92uxe2x88x92sAuCunOwxe2x80x83xe2x80x83(2)
where L is any of or any combination of Pb, Sb, or Ti, and 0xe2x89xa6xxe2x89xa60.4.
More preferred materials of the invention are those of formula (2) where nxe2x89xa71 and 0xe2x89xa6exe2x89xa60.4 and having the formula
Bi2+exe2x88x92xLxCan+yxe2x88x921xe2x88x92sSr2xe2x88x92yAzCunO2n+4+dxe2x80x83xe2x80x83(3)
and where 0xe2x89xa6zxe2x89xa60.4, xe2x88x922xe2x89xa6yxe2x89xa62, and xe2x88x921xe2x89xa6dxe2x89xa61.
Materials or formula (3) of the invention wherein n is 3 have the formula
Bi2+exe2x88x92xLxCa2+yxe2x88x92sSr2xe2x88x92yAzCu3O10+d.xe2x80x83xe2x80x83(4)
Preferably in the n=3 materials of formula (4) L is Pb and 0xe2x89xa6xxe2x89xa60.4 and xe2x88x921xe2x89xa6y, dxe2x89xa61. A may preferably be Y or Na and 0xe2x89xa6zxe2x89xa60.4 and preferably 0, with preferably 0.5xe2x89xa6yxe2x89xa60.5 and xe2x88x921xe2x89xa6dxe2x89xa61. Preferably d is fixed in a range determined by annealing in air at between 300xc2x0 C. and 550xc2x0 C., or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to.annealing in air at between 300xc2x0 C. and 550xc2x0 C. Most preferably 0.2xe2x89xa6e, sxe2x89xa60.3, 0.3xe2x89xa6xxe2x89xa60.4 and xe2x88x920.1xe2x89xa6yxe2x89xa60.1.
Especially preferred n=3 materials of the invention are Bi1.9Pb0.35Ca2Sr2Cu3O10+d, Bi2.1Ca2Sr2Cu3O10+d, preferably wherein d is fixed in a range determined by annealing in air at between 300xc2x0 C. and 550xc2x0 C., or by annealing in an atmosphere at any oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 300xc2x0 C. and 550xc2x0 C.
Materials of formula (3) of the invention wherein n is 2 have the formula
Bi2+exe2x88x92xLxCa1+yxe2x88x92sSr2xe2x88x92yAzCu2O8+dxe2x80x83xe2x80x83(5)
where xe2x88x921xe2x89xa6dxe2x89xa61.
Preferably in the n=2 materials of formula (5) L is Pb and 0 less than x and most preferably 0 less than xxe2x89xa60.4, z is 0, and xe2x88x921xe2x89xa6y, dxe2x89xa61. A may preferably be Y or Na; where A is Y preferably 0 less than zxe2x89xa60.4 and most preferably 0 less than zxe2x89xa60.1 and where A is Na preferably 0 less than zxe2x89xa60.4, x is 0, and xe2x88x921xe2x89xa6y, dxe2x89xa61; in both cases preferably d is fixed in a range determined by annealing in air at between 700xc2x0 C. and 830xc2x0 C. or in 2% oxygen at between 600xc2x0 C. and 800xc2x0 C., or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 700xc2x0 C. and 800xc2x0 C.
A preferred n=2 material is that of formula (5) wherein L is Pb, and where 0 less than xxe2x89xa60.4, z is 0, 0xe2x89xa6e, sxe2x89xa60.25, y is xe2x88x920.5, and d is fixed in a range determined by annealing the material in air between 600xc2x0 C. and 800xc2x0 C., or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 700xc2x0 C. and 800xc2x0 C.
A further preferred n=2 material is that of formula (5) wherein L is Pb, and where 0 less than xxe2x89xa60.4, z is 0, 0xe2x89xa6e, sxe2x89xa60.25, y is 0, and d is fixed in a range determined by annealing in air at between 700xc2x0 C. and 830xc2x0 C. or in 2% oxygen at between 600xc2x0 C. and 800xc2x0 C. or by annealing in an atmosphere and at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 700xc2x0 C. and 830xc2x0 C.
A further preferred n=2 material is that of formula (5) wherein L is Pb, and where 0xe2x89xa6e, sxe2x89xa60.25, 0xe2x89xa6xxe2x89xa60.4, y is 0.5, z is 0, and d is fixed in a range determined by annealing in air at between 450xc2x0 C. and 700xc2x0 C. or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 450xc2x0 C. and 700xc2x0 C.
A further preferred n=2 material is that of formula (5) having the formula Bi2.1+ePb0.2Ca1xe2x88x92sSr2Cu2O8+d and where 0xe2x89xa6e, sxe2x89xa60.2 and d is fixed in a range determined by annealing in 2% oxygen at between 770xc2x0 C. and 830xc2x0 C. or in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in 2% oxygen at between 770xc2x0 C. and 830xc2x0 C.
A further preferred n=2 material is that of formula (5) having the formula Bi2+eCa1+yxe2x88x92sSr2xe2x88x92yCu2O8+d where xe2x88x920.5xe2x89xa6yxe2x89xa60.5 and 0 less than e, sxe2x89xa60.25 and most preferably wherein y is xe2x88x920.5 or most preferably 0, and d is fixed in a range determined by annealing in air at between 600xc2x0 C. and 800xc2x0 C. or by annealing in an atmosphere at an oxygen partial pressure and temperature equivalent to annealing in air at between 600xc2x0 C. and 800xc2x0 C.; of the above formula or wherein y is 0.5, and d is fixed in a range determined by annealing the material in air at between 450xc2x0 C. and 700xc2x0 C. or in an atmosphere other than air at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 450xc2x0 C. and 700xc2x0 C.
Materials of formula (1) of the invention wherein n is 1 have the formula
Bi2+exe2x88x92xLxCayxe2x88x92sSr2xe2x88x92yAzCuO6+dxe2x80x83xe2x80x83(6)
where 0xe2x89xa6yxe2x89xa61 and xe2x88x921xe2x89xa6dxe2x89xa61
Preferably in the n=1 materials of formula (6) L is Pb and 0xe2x89xa6xxe2x89xa60.4. A is preferably Na and 0 less than zxe2x89xa60.4.
A preferred n=1 material is that of formula (6) wherein L is Pb, and where 0 less than xxe2x89xa60.4, 0xe2x89xa6e, sxe2x89xa60.25 y is 0.7, and z is 0.
A further preferred n=1 material is that of formula (6) wherein L is Pb, and where 0xe2x89xa6e, sxe2x89xa60.25, 0xe2x89xa6xxe2x89xa60.4, y is 1 and z is 0.
A further preferred n=1 material is that of formula (6) wherein L is Pb, and where z is 0, 0xe2x89xa6e, sxe2x89xa60.25 0.3xe2x89xa6xxe2x89xa60.4, and 0.5xe2x89xa6yxe2x89xa60.7.
A preferred n=1 material is
Bi1.85Pb0.35Ca0.4Sr1.4CuO6+d.
A preferred n=1 material is Bi2+eCayxe2x88x92sSr2xe2x88x92yCuO6+d where 0 less than e, sxe2x89xa60.25 and y is 1 and d is fixed in a range determined by annealing in air at between 300xc2x0 C. and 500xc2x0 C. or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 300xc2x0 C. and 500xc2x0 C.
A further preferred n=1 material is that of formula (6) where y is 0.67, and d is fixed in a range determined by annealing in air at between 400xc2x0 C. and 600xc2x0 C. or by annealing in an atmosphere at an oxygen pressure or partial pressure and temperature equivalent to annealing in air at between 400xc2x0 C. and 600xc2x0 C.
Materials of formula (1) of the invention where n is 0 have the formula
Bi2+exe2x88x92xLxA1xe2x88x92zAxe2x80x2zOwxe2x80x83xe2x80x83(7)
where 0xe2x89xa6xxe2x89xa60.4 and 0xe2x89xa6zxe2x89xa61 with the proviso that z is not 0 when x is 0, and where 0xe2x89xa6exe2x89xa64. A1xe2x88x92zAxe2x80x2z is the combinational form of A as defined for formula (1).
Preferably in the n=0 materials of formula (7) wherein L is Pb and preferably A is Ca, Sr, or Ba or any combination thereof. Where A is Ca preferably Axe2x80x2 is Sr, Ba, Na or Y, or any combination thereof. Where A is Sr preferably Axe2x80x2 is Ba, Na or K, or any combination thereof. Most preferably A is Ba and Axe2x80x2 is K.
Specific materials of the invention include
Bi2+eSr0.8Na0.2O4+d, Bi2+eBa0.5K0.2O4+d,
Bi2+eCa0.5Sr0.5O4+d, Bi2+eBa0.5Sr0.5O4+d,
Bi1.9+ePb0.1SrO4+d, and Bi1.9+ePb0.1BaO4+d, with 0xe2x89xa6exe2x89xa64 and 3xe2x89xa6dxe2x89xa610.
The invention encompasses materials of formula (7) wherein nxe2x89xa72, s=0 and M is Cu or Cu substituted by any of or any combination of Bi, Sb, Pb, Ti or any other transition metal, and having the formula
Rnxe2x88x921A2CumCunOwxe2x80x83xe2x80x83(8)
Preferably in materials of formula (8) of the invention A is Ba, m is 1 or 2, and n is 3, 4, . . . A material of this class may be where m=3/2 and n=2, and having the formula
RBa2Cu3.5O7+d
where xe2x88x920.5xe2x89xa6dxe2x89xa60.5, and where R is Y.
The invention encompasses a material having the formula
Ynxe2x88x921Ba2Cun+1O(5/2)n+3/2+d
where xe2x88x921xe2x89xa6dxe2x89xa61 and n is 3, 4, 5, . . . ,
a material having the formula
Ynxe2x88x921Ba2Cun+2O(5/2)n+5/2+d, and,
a material having the formula
RBa2Canxe2x88x922Cun+2O(5/2)n+5/2+d
wherein n is 3, 4, 5, . . . preferably where R is Y.
The materials of the invention may be formed as mixed phase or intergrowth structures incorporating structural sequences from a number of the above described materials also including sequences from the material RBa2Cu3O7xe2x88x92d and its derivatives. For example, this includes RBa2Cu3.5O7.5xe2x88x92d comprising, as it does, approximately alternating sequences of RBa2Cu3O7xe2x88x92d and RBa2Cu4O8xe2x88x92d. This also includes materials with general formula (1) with n taking non-integral values allowing for the fact that, for example, a predominantly n=2 material may have n=1 and n=3 intergrowths. This also includes materials with general formula (1) with n taking non-integral values allowing for ordered mixed sequences of cells of different n values, for example, n=2.5 for alternating sequences of n=2 and n=3 slabs.
Typically, the materials of the invention may be prepared by solid state reaction of precursor materials such as metals, oxides, carbonates, nitrates, hydroxides, or any organic salt or organo-metallic material, for example, such as Bi2O3, Pb(NO3)2, Sr(NO3)2, Ca(NO3)2 and CuO for BiPbSrCaCuO materials. The materials of the invention may also be prepared by liquid flux reaction or vapour phase deposition techniques for example, as will be known to those in the art. Following forming of the materials oxygen loading or unloading as appropriate to achieve the optimum oxygen stoichiometry, for example for superconductivity, is carried out. The above preparation techniques are described in xe2x80x9cChemistry of High Temperature Superconductorsxe2x80x9dxe2x80x94Eds. D L Nelson, M S Whittingham and T F George, American Chemical Society Symposium Series 351 (1987); Buckley et al, Physica C156, 629 (1988); and Torardi et al Science 240, 631 (1988), for example. The materials may be prepared in the form of any sintered ceramic, recrystallised glass, thick film, thin film, filaments or single crystals.
In order to achieve maximum strength and toughness for the materials, it is important that they are prepared to a density close to the theoretical density. As prepared by common solid-state reaction and sintering techniques, densities of about 80% theoretical density can readily be achieved. Higher densities may be achieved by, for example, spray drying or freeze drying powders as described for example in Johnson et al, Advanced Ceramic Materials 2, 337 (1987), spray pyrolysis as described for example in Kodas et al, Applied Physics Letters 52, 1622 (1988), precipitation or sol gel methods as described for example in Barboux et al J. Applied Physics 63, 2725 (1988), in order to achieve very fine particles of dimension 20 to 100 mm. After die-pressing these will sinter to high density. Alternatively, to achieve higher densities one may hot press, extrude, or rapidly solidify the ceramic material from the melt after solid state reaction or grow single crystals.
Preparation of the materials of the invention may be carried out more rapidly if in preparation of the materials by solid state reaction of precursor material any or all of the cations in the end material are introduced as precursors in the nitrate or hydroxide forms for rapid reaction of bulk material in the nitrate or hydroxide melt. Both the temperature and duration of the preparation reaction may be lowered by using nitrate or hydroxide precursors to introduce the cations. Melting of the nitrate and/or hydroxide precursors allows intimate atomic mixing prior to decomposition and efflux of oxides of nitrogen.
After preparation the materials may be sintered or (re-)ground to small particles and pressed to shape and sintered as desired to form the end material for use, as is known in the art and/or annealed to relieve stresses and increase strength and toughness as is similarly known in the art for the unsubstituted materials. The materials after preparation may as necessary be loaded on unloaded with oxygen to achieve the optimum stoichiometry for superconductivity, optimised oxygen mobility, or other material properties. As stated, for n=2 BCSCO materials for example this generally requires oxygen unloading into the materials and with known technique this is generally carried out by annealing at 700xc2x0 C. to 830xc2x0 C. in air over 1 to 4 hours followed by rapid quenching into liquid nitrogen, for example. Most suitably, oxygen loading or unloading is carried out during cooling from the reaction temperatures immediately after the preparation reaction, where the materials are prepared by solid state reaction for example. Alternatively and/or additionally oxygen loading may be carried out during sintering or annealing in an oxygen containing atmosphere at an appropriate pressure or partial pressure of oxygen. Without loss of generality the materials may be annealed, cooled, quenched or subjected to any general heat treatment incorporating AgO or Ag2O as oxidants or in controlled gaseous atmospheres such as argon, air or oxygen followed by rapid quenching so as to control the oxygen stoichiometry of the novel materials, the said stoichiometry being described by the variables w or d. The materials may be used as prepared without necessarily requiring oxygen loading or unloading for forming electrodes, electrolytes, sensors, catalysts and the like utilising high oxygen mobility property of the materials.